Optical fibers are very suitable for data transmission because of their known properties of high signal speed, structural simplicity and low distortion. They also compare favorably with metallic transmission lines such as coaxial cables by their low attenuation, wide pass band and efficient shielding. They are generally used in conjunction with sources of luminous energy such as lassers or light-emitting diodes (LEDs) which can be readily pulsed but are difficult to modulate in amplitude with more than two distinct levels, i.e. low (L) and high (H).
The use of simple binary pulse trains for data transmission over any kind of signal path, optical or electrical, has certain drawbacks especially when the pulse rate must be reconstituted at the receiving end by the synchronization extractor. Thus, the rhythm may be lost upon the occurrence of a long series of identical pulses of logical value "0" or "1", also, since any bit combination has a specific numerical value, there is no way of detecting transmission errors from the configuration of the pulse train, in contrast to ternary or quaternary pulse trains which may provide a certain amount of redundancy but are unsuitable for clestro-optical systems. The frequency spectrum of a binary pulse train has a large d-c component and its energy is concentrated near the lower end of that spectrum. This causes considerable intersymbol interference if the input stage of the receiver is not equipped to handle direct current; in the case of a photodetector forming part of that receiver, as is necessary with fiber-optical transmission, short-term fluctuations of the d-c component may load that detector with transient biasing voltages giving rise to further distortions.